Project Summary/Abstract Soft tissue sarcomas are an aggressive group of mesenchymal malignancies diagnosed in 200,000 people per year worldwide. Unlike in epithelial cancers, where novel targeted therapies have had a dramatic effect on patient survival, the treatment approach for mesenchymal tumors including sarcomas has not changed significantly in 25 years. Our recent work revealed that deregulation of the Hippo pathway enhances sarcomagenesis in the aggressive muscle tumor, Undifferentiated pleomorphic sarcoma (UPS). UPS is a commonly diagnosed and metastatic sarcoma subtype frequently found in adult muscle tissues. We have observed that loss of Angiomotin (AMOT), a crucial mediator of Hippo-associated growth restriction, is required for UPS sarcomagenesis. AMOT is highly expressed in differentiated human muscle tissue but is silenced in UPS and other sarcomas. Ectopic re-expression of the p130 isoform of AMOT significantly inhibits sarcoma cell proliferation in vitro. This finding is consistent with the only known function of AMOT in cancer cells, which is to sequester the Hippo pathway effector YAP1 and facilitate its degradation. YAP1 is a pro-proliferation transcriptional regulator whose deletion in an autochthonous mouse model of UPS significantly decreased tumorigenesis. Together these data suggest that AMOT loss promotes YAP-mediated sarcomagenesis in muscle-derived UPS. We next investigated the downstream effects of YAP1 expression in UPS by microarray gene expression studies of control and Yap1-deficient murine tumors. We found that Yap1 controls NF-?B signaling in UPS by suppressing expression of Usp31, a negative regulator of NF-?B activity. Furthermore, using ChIP-seq of patient samples we found that NF-?B signaling is substantially upregulated in human UPS. Consistent with these findings, UPS cell proliferation is highly sensitive to NF-?B inhibition. Based on these findings in Specific Aim1 we will determine how AMOT loss is controlled in UPS and if this process is required for tumor initiation in soft tissue sarcomas. Next we will define the mechanism by which YAP1 suppresses USP31 expression in Specific Aim 2. We will investigate whether YAP1 directly binds to the promoter region of USP31, preventing its transcription. Loss of YAP1 restores expression of USP31, a peptidase that removes activation specific ubiquitin modifications from lysine 63 in TRAF molecules upstream of p65, thereby inactivating NF-?B. The role of NF-?B in normal skeletal muscle progenitors, the putative cell of origin of UPS, is to promote proliferation and prevent differentiation. In Specific Aim3 we will determine which YAP1- dependent NF-?B targets are necessary for regulating either or both of these processes. The goal of this proposal is to test the hypothesis that deregulated Hippo signaling promotes sarcomagenesis via suppression of AMOT, resulting in YAP1/NF-?B-associated proliferation and inhibition of differentiation. Ultimately, the purpose of these studies is to identify clinically actionable therapeutic targets to advance treatment for skeletal muscle UPS patients.